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To define our sprites, first add the
following constant declarations at the top of Form1's code
window:
Const SPRITE_SHIP = 0
Const SPRITE_BULLET = 1
Const SPRITE_BUZZARD_FIRST = 2
Const NUM_BUZZARDS = 3
Using constants like this for sprite
identifiers will help keep the game code more readable.
Now we'll use the SpriteDefine method to create our sprites.
SpriteDefine has the following signature:
SpriteDefine(lSpriteNum As Long, sFileName
As String, _
iXPosBMP As Long, iYPosBMP As Long, _
iFrameWidth As Long, iFrameHeight As Long, _
iFrameCount As Long, iFramesPerRow As Long)
Let's discuss these parameters a bit,
as SpriteDefine is a core method in ASpriteCE: lSpriteNum
is a distinct identifier for the sprite. You should start
at zero for your first sprite and increment from there.
sFileName is the absolute path to the image. iXPosBMP and
iYPosBMP are the coordinates in the image file that the
sprite frames begin at. This is useful only if your image
file contains multiple sprites, otherwise simply pass zeros
for these. iFrameWidth and iFrameHeight define the dimensions
of each frame for the sprite. iFrameCount is the number
of frames the sprite has. Finally, iFramesPerRow is equal
to the number of frames on each "row" in the image
file; this is equal to iFrameCount unless you create your
image file by breaking frames into multiple rows (useful
for sprites with a lot of frames).
OK, now let's implement SpriteDefine
for our sprites:
Game1.SpriteDefine SPRITE_SHIP, _
App.Path & "\ship.gif", 0, 0, 28, 35, 1, 1
Game1.SpriteDefine SPRITE_BULLET, _
App.Path & "\bullet.gif", 0, 0, 5, 8, 1, 1
Game1.SpriteDefine SPRITE_BUZZARD_1, _
App.Path & "\buzzard.gif", 0, 0, 36, 16, 5,
5
In DevBuzzards, we will want to have
multiple buzzard sprites that have identical source frames.
If we were to use SpriteDefine to create all of the buzzards,
we would be wasting memory by duplicating the frame data
for each one. Instead, we can use the SpriteCopy method
which efficiently copies sprites by sharing their frame
data:
' Make copies of buzzard sprite
Dim i As Integer
For i = SPRITE_BUZZARD_1 + 1 To _
SPRITE_BUZZARD_1 + NUM_BUZZARDS - 1
Game1.SpriteCopy SPRITE_BUZZARD_1, i
Next
Next we'll position the sprites with
a call to SpriteSetPosition - this is pretty straightforward.
The SpriteFrameRate property will be used to make the buzzards
animate one sprite frame for every three game frames. We
also want to make the bullet sprite initially inactive by
using the SpriteActive property.
' Position sprites
Game1.SpriteSetPosition SPRITE_SHIP, 0, 150
For i = SPRITE_BUZZARD_1 To _
SPRITE_BUZZARD_1 + NUM_BUZZARDS - 1
Game1.SpriteSetPosition i, _
(i - SPRITE_BUZZARD_1) * 40, 10
Game1.SpriteFrameRate(i) = 3
Next
Game1.SpriteActive(SPRITE_BULLET) = False
The last part of our "setup"
code in Form_Load will start the game timer with a call
to StartTimer. The timer determines the frame rate
of our game, and in DevBuzzards we'll aim for 20 frames
per second. We pass StartTimer a value of the number of
milliseconds between game frames - 50 milliseconds will
give us 20 frames per second.
' Start Timer
Game1.TimerStartTimer 50
At this point, we've only written about
a dozen lines of code, but it's interesting to see how much
this code does! Go ahead and make the VB file by selecting
File/Make Project1.vb from the eVB menu. Then, copy the
Project1.vb file along with the four image files to a directory
on your device and run Project1.vb. You should see the animated
buzzard sprites at the top and the deVBuzz PDA logo at the
bottom, with the starfield image in the background.
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